The Office of the Director, Division of Cancer Etiology, for many years, has had a contract to perform mutagenicity assays (Mouse Lymphoma L5178Y TK+/-) in support of extramural and intramural activities of the Division, with particular emphasis on providing in vitro data for use in the NCI chemical selection process of chemicals nominated, for carcinogenicity testing, to the National Toxicology Program. Since there is often a paucity of in vitro data on chemicals under consideration, the information obtained from these assays is most important in permitting the members of the Chemical Selection Working Group (CSWG) to make informed decisions in a timely fashion. The objective of this procurement is to perform Mouse Lymphoma L5178Y TK+/- assays and report on between 20-30 compounds per year, with about one third of the compounds requiring an additional assay before the final report is written.